The common rule of thumb is 24 hours. However, I only use that method only if problems keep coming up after a few hours of scanning with clean results. Most of my faulty RAM failed within the first pass.

I think it would depend on your circumstances and how much time you can devote for testing. A new build I've settled for running it 4 or 5 times for each stick. Suspected memory problems on an older system, I pull all memory, inspect, re-insert and test one at a time more thoroughly, then test them all together.

When overclocking RAM, I usually let it complete 4-5 passes and then check with Prime95 large FFT's. Oddly, I've had RAM that passes Memtest with flying colours, but fails Prime95 large FFT's within a few seconds. So, that's why I usually like to have a second opinion...just some food for thought. :)

1 or 2. If it's gonna fail it will almost always fail first pass (with lots of errors) Sometimes when working with cheaper ram, if it does 1 or 2 passes successfully and then starts to get an error or 2 per pass, it's just over heating, when really it's fine.